Archean high-Mg quartz-monzodiorite suite: A re-evaluation of the parental magma and differentiation

Quartz-monzodiorite is generally considered to represent the parent magma of high-Mg quartz-monzodiorite plutonic suites; thus such suites are thought to be melts from a metasomatized mantle wedge. It is proposed here that hornblende tonalite, which is present in the feeder dikes of high-Mg quartz-monzodiorite plutonic suites, better represents the parental magma. Hornblende tonalite has a higher compatible element content with a relatively lower incompatible element content and a lower (La/Yb)(n) ratio than quartz-monzodiorite, suggesting that the tonalite is the more primitive magma of the suite. The hornblendite unit of the high-Mg quartz-monzodiorite plutonic suite is interpreted to represent the cumulate, and the quartz-monzodiorite the differentiated liquid. To test this hypothesis and explain the differentiation of the units of a high-Mg quartz-monzodiorite plutonic suite, the Houghton pluton in the northern Abitibi greenstone belt, Quebec, was studied. Results show that crystal fractionation and equilibrium crystallization cannot sufficiently enrich incompatible elements in the quartz-monzodiorite while keeping the compatible element contents high. Feeder dike petrography suggests that the magma chamber was periodically replenished. The model that best explains the formation of the different units of this suite is a periodically replenished magma chamber with a crystallization sequence of hornblende followed by hornblende and plagioclase. Such a magma chamber undergoing crystal fractionation will produce the high incompatible and compatible element contents typical of quartz-monzodiorite magmas. Not all quartz-monzodiorite magmas of the site can be explained by a replenishment of the hornblende tonalite. As identified in the feeder system, the replenishing magma changed at some time to one represented by leucotonalite-to-quartz-diorite. By changing the replenishing magma from the hornblende tonalite to the leucotonalite-to-quartz-diorite unit, after the differentiated liquid reached 64% SiO2, it is possible to model all the units in the plutonic suite. The hornblendite unit can be easily modeled for the major oxides, but the inclusion or exclusion of accessory phases rich in trace elements (allanite) in the samples renders trace element behavior erratic, and the hornblendite cannot be successfully modeled.